Infection with oncogenic retroviruses requires cell division, while infection with lentiviruses does not. The basis for these differences in poorly understood, and has important implications for designing viral vectors. The current gene therapy vectors based on murine leukemia virus (MLV) are severely limited by their requirement for cell division, especially in treating the many hematopoietic diseases potentially curable by gene transfer into non-dividing stem cells. Possible explanations for the poor infection of non-dividing cells by MLV include low receptor levels, incomplete reverse transcription, and a block in provirus integration. The experiments described in this proposal address these different possibilities, and compare examples of oncornaviruses, lentiviruses, and foamy viruses, which are the third and least studied class of retroviruses. Vectors based on MLV, visna virus and human foamy virus (HFV) encoding neomycin resistance or alkaline phosphatase will be packaged in identical envelope proteins, and used to infect dividing and stationary phase fibroblasts. Transduction frequencies, viral DNA synthesis, and provirus integration will be measured, thereby comparing all three classes of retroviruses in the same experimental system. It is hoped that visna or HFV vectors will be capable of transducing non-dividing cells, and that improved vector packaging lines can be constructed based on these viruses.